Internally insulated extrusion die

ABSTRACT

A die assembly for plastic pelletizing apparatus of the type including a cooling bath into which the plastic is extruded. The die assembly includes a heated main body having first and second oppositely disposed exterior faces connected by a plurality of relatively small extrusion passageways. A layer of thermal insulation material overlies the second face, and a cover member having an outer surface adapted to be exposed to the cooling bath is positioned over the thermal insulation material and joined to the main body. Wear resistant insert members formed from a material having a higher coefficient of thermal conductivity than the material from which the cover member is made extend from the outer surface of the cover member through the layer of insulation into the main body. Extrusion outlet orifices are formed through the insert members and define continuations of the extrusion passageways of the main body. The relationship between the inserts, the cover, the insulation, and the main body produces an increased heat flow from the main body to the outer die face in confined areas about the orifices. This heat flow advantageously reduces the possibility of freeze-off and plugging of the orifices due to premature solidification of the extruded plastic.

BACKGROUND OF THE INVENTION

The subject invention is directed to the art of extrusion dies and, moreparticularly to an extrusion die of the type used for extrusion ofsynthetic resins and plastics.

The invention is particularly suited for use in an underwater type,plastic pelletizing apparatus and will be described with referencethereto; however, the invention is capable of broader application andcould be adapted for use in other environments and for other purposes.

One of the more common apparatuses used for pelletizing plasticmaterials comprises a rotary extruder which extrudes the resin orplastic material in a molten or fluid state through an extrusion diehaving a multiplicity of small diameter extrusion passages. Typically,the face of the die is submerged in a cooling bath such that the plasticmaterial solidifies just as it is leaving the die. As the material exitsfrom the face of the die, it is cut into pellets or granules by arotating knife passing over the die face.

A continuous and ongoing problem with the foregoing apparatus has beenthe difficulty of assuring that the material does not solidify duringits passage through the die. As is apparent, premature solidification ofthe material can result in plugging or "freezing-off" of the extrusionpassages and/or outlet orifices.

In an effort to maintain the material molten during its passage throughthe die, the dies have typically been designed with internal heatingpassages through which steam or heated oil has been circulated. Inaddition, various arrangements have been used to retard the flow of heatfrom the body of the die to the cooling bath. For example, layers ofinsulation have been placed over the exposed faces of the die, orbetween the die face and the main die body. Alternatively, the diepassages have been provided with insulating sleeves of ceramic or thelike to retard heat flow from the molten plastic to the die body orface.

While the above mentioned approaches have alleviated the difficulty with"freeze-off" to some extent, it still remains an ongoing problem. Tothat end, the subject invention provides an extrusion die constructionwhich is relatively simple while operating to significantly reduce"freeze-off" and provide extended surface wear life.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, a new and improved extrusiondoe construction is provided wherein a heated main die body having aplurality of plastic extrusion passageways extending through the body toan outer face is provided with a layer of thermal insulation over theouter face and a cover member which overlies and encloses the insulationis joined to the main body. Wear resistant insert members formed from amaterial having a coefficient of thermal conductivity significantlyhigher than the material from which the cover is made extend through thecover member and the insulation into the main die body. Each of theinsert members includes an extrusion outlet orifice which forms acontinuation of an associated extrusion passageway in the main body.

The use of the insert members with their relatively higher coefficientof thermal conductivity increases the heat flow from the main body tothe outer surface of the assembly in the area closely surrounding theorifices. This permits the temperature of the orifices to be maintainedat a higher level to thereby prevent freeze-off without requiring asignificant increase in the heat input to the die body. The remainder ofthe exposed cover member or face is, however, at a relatively lowertemperature because of the insulation.

In accordance with a further aspect of the invention, the outer endfaces of the insert members are preferably located in a common planecoextensive with the outer face of the cover member or outwardlythereof. The wear resistant insert members can thus define the surfaceover which the knife or knives travel to pelletize the extruded plasic.

In accordance with a still further aspect of the invention, the insertmembers are preferably of cylindrical shape with the outlet orificesformed axially therethrough. In addition, the inserts are preferablybonded to both the main body and the cover member, such as by brazing.

The principal advantage of the subject invention is the provision of anew and improved extrusion die which is arranged to eliminate theproblem of freeze-off while providing a highly wear resistant face.

A further advantage resides in the provision of an extrusion die of thegeneral type described that is internally insulated to reduce heat lossto the outer face, but which is provided with means to increase heatflow in the areas about the outlet orifices.

Other benefits and advantages of the subject invention will becomeapparent to those skilled in the art upon a reading and understanding ofthe following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, a preferred embodiment of which will be described in detail inthis specification and illustrated in the accompanying drawings whichform a part hereof and wherein:

FIG. 1 is a front elevational view of an extrusion die formed inaccordance with a preferred embodiment of the invention;

FIG. 2 is an enlarged view of the portion of FIG. 1 enclosed by adash-dot line; and,

FIG. 3 is a cross-sectional view taken generally along lines 2--2 ofFIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the invention only and not forthe purpose of limiting same, FIGS. 1-3 show an extrusion die assemblycomprising a main die body A and a top plate or cover member B which isconnected to the forward end face of the main die body A.

The subject extrusion die assembly is adapted for use with aconventional extruder apparatus which is not shown inasmuch as it doesnot comprise a part of the present invention and is not necessary to anappreciation thereof. In using the subject invention in its preferredenvironment, a synthetic resin or polymeric plastic material in moltenor fluid form is extruded from the extruder through main die body A andoutwardly through cover or top plate member B. The interior of the diebody is maintained at an elevated temperature in a manner subsequentlyto be described.

The outer surface of cover member B is normally disposed in a coolingbath of water or other liquid maintained at a temperature substantiallylower than the temperature of the interior of main die body A. As thesynthetic resin or polymeric plastic material in a liquid or fluid stateis extruded through the main die body A and the cover member B, it iscooled to a solid state as it discharges into the cooling bath. As thesolidified material enters the cooling bath, it is cut into pellets by aknife or knives which periodically wipe across the outer surface ofcover member B. This general process is well known in the art and iscommonly referred to as underwater pelletizing.

More particularly, and with reference to FIGS. 1-3, main die body A isdefined by a generally cylindrical body 10 formed from any suitablematerial, e.g., stainless steel, and provided about its outer peripherywith a plurality of counterbored, bolt receiving openings 12 which allowthe body 10 to be suitably connected to the associated extruder (notshown). In addition, a plurality of similarly counterbored openings 16are formed generally axially through body 10 adjacent the center areathereof to also provide for mounting of the assembly.

As best shown in FIG. 3, body 10 includes a pair of oppositely disposed,generally parallel end faces 18 and 20. End face 18 is arranged to beconnected to the discharge end of the extruder to receive the molten orfluid resin or plastic material coming therefrom. As shown, amultiplicity of relatively closely spaced, generally parallel extrusionpassageways 22 extend in the axial direction through body 10. In theembodiment under consideration, all of the extrusion passageways 22 aregenerally identical in configuration and include a tapered inlet endopening 24 with an enlarged, generally cylindrical passageway section26. The inner end of section 26 is provided with a tapered passagewaysection 28 leading to the outlet section 30 which extends through face20.

Heating means is provided to maintain the body 10 at an elevatedtemperature sufficient to prevent solidification of the molten or fluidmaterial during its travel through passageways 22. In the subjectembodiment, the heating means comprises a plurality of fluid passages 34formed transversely of the body intermediate aligned rows of passageways22. The passages 34 are sealed from passageways 22 and function toconduct a heated fluid such as steam or oil transversely through the diefrom an inlet 36 to an outlet 38 (see FIG. 1). It should, of course, beunderstood that the opposite ends of passages are suitably connected tothe inlet and outlet 36, 38 by internal header passages (not shown).

In order to prevent excess heat flow from the main body outwardlythrough end face 20 to the associated cooling bath, the subject dieconstruction includes a layer of suitable thermal insulation material 40overlying end face 20. Preferably, the insulation 40 substantiallycompletely covers the end face 20 throughout the area under cover memberB. As can be appreciated, the particular type of insulation used is notcritical to the subject invention and may, for example, comprise aceramic-type material or any other material which is suitable for hightemperature applications and has desirable insulating characteristics.In the embodiment shown, zirconium oxide is preferred and can be sprayedor otherwise applied to surface 40. Additionally, the thickness of theinsulation can vary as desired or necessary to provide the neededinsulation characteristics.

In the illustrated embodiment, the insulation material 40 is sealed fromdirect contact with the cooling bath by the top plate or cover member B.As best seen in FIGS. 1 and 3, cover member B comprises a generallyflat, annular shaped disk member 44 which is positioned concentricallyon end face 20 of body 10. The member 44 is preferably positioned indirect contact with insulation 40, and is rigidly and sealingly joinedto body 10 such as by circumferential inner and outer weld beads 46 and48.

The body 10 and the cover member 44 are preferably formed from metalshaving suitable temperature and corrosion resistance as required by theoperating environment. For example, precipitation hardening stainlesssteels have been used successfully in die assemblies of the type underconsideration.

Of particular importance to the subject invention is the arrangement forconnecting the outlet ends of passageways 22 with the outer face 50 ofcover member 44. In particular, insert members 52 extend inwardly fromouter face 50 through the cover member 44 and the insulation layer 40into recesses 42 in the main body 10. Each of the inserts 52 preferablyhas a generally cylindrical configuration with an axially extendingcenterbore 54. In addition, the insert members 52 are formed from ahard, wear resistant material capable of withstanding the abrasioncharacteristics of the extruded material and the wear induced by theknives traveling over their exposed ends. Moreover, the insert membersmust be formed from a material having a coefficient of thermalconductivity substantially greater than that of the cover member 44. Inthe preferred embodiment, the insert members 52 are formed from sinteredtungsten carbide having a coefficient of thermal conductivity of about0.19 cal/cm/sec/°C. A typical precipitation hardening stainless steel asused for the cover member 44 has a coefficient of thermal conductivityof about 0.050 cal/cm/sec/°C. Other materials having suitablecharacteristics could, of course, also be used.

One of the insert members 52 is preferably associated with each of theextrusion passageways 22 and defines the outlet discharge orifice forthe associated passageway 22. As best shown in FIG. 3, the centralopening 54 of the inserts 52 is preferably sized to generally correspondto the diameter of section 30 of the extrusion passageways 22. Inaddition, each of the insert members 52 is desirably permanently bondedsuch as by brazing or the like to both the body 10 and the cover member44.

By directly connecting the insert members 52 through the insulationlayer 40 and into thermal engagement with the body 10, heat flow fromthe body 10 to the outer face 50 is significantly increased in the areaclosely surrounding each of the discharge orifices 54. This has theeffect of raising the temperature of the outlet orifices and preventingfreeze-off or solidification of the plastic material passingtherethrough.

In addition to the above, it is preferred that the outer end face ofeach insert 52 lie in a common plane which is preferably in the plane offace 50 or slightly outwardly thereof. The outer ends of the inserts 52can thus define the surface over which the knives travel in pelletizingthe plastic which solidifies as it exits through the outer end of thedischarge orifices 54.

Preferably, and as best shown in FIG. 2, the discharge passageway 30and/or the associated inserts 52 should be laid out such that they arenot in circumferentially aligned rows. That is, the wear surfacesprovided by the inserts 52 should be relatively evenly distributed atvarying radial distances to assure that wear on the knife or knives andthe inserts is uniform.

It should be noted that, under certain conditions, the layout of theextrusion passageways 22 is such that the pattern cannot be completelyuniform about the outer face of the die assembly. Under suchcircumstances, it is sometimes desirable to provide a more even wearsurface to the outer face than can be achieved by the insert membersalone. To that end, the subject invention contemplates the use ofsuitable wear pads or buttons 58 (see FIGS. 2 and 3) positioned in coverplate member 44 at a location so as to provide a more even distributionof wear areas. Preferably, the wear pads 58 are generally cylindricaland are formed from a suitable, wear resistant material. They arepositioned in recesses 60 formed in the cover plate 44 such that theouter or exposed surface of each wear pad 58 lies in the same plane asthe outer exposed surfaces of the inserts 52. It is important to note,however, that the wear pads 58 should not extend through the covermember 44 or the insulation layer 40. In addition, it is preferable thatthe wear pads 58 do not engage the inserts 52. The reason for this isthat if engagement with the main die body or the inserts were to takeplace, the wear pads 58 would cause an increased heat flow to thecooling bath with no benefit to the operation of the die assembly.

The invention has been described with reference to the preferedembodiment. Obviously, modifications and alterations will occur toothers upon a reading and understanding of this specification. It isintended to include all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof.

Having thus described the invention, it is now claimed:
 1. An extrusiondie assembly for plastic pelletizing apparatus of the type including acooling bath into which plastic pellets are extruded comprising:a mainbody having first and second oppositely disposed exterior faces; aplurality of plastic extrusion passageways extending through said bodybetween said first and second faces; a layer of thermal insulationmaterial overlying said second face; means for applying heat to saidmain body; a cover member positioned over said thermal insulationmaterial and joined to said main body, said cover member having an outersurface exposed to said cooling bath and having a plurality of recessestherein; wear resistant insert members formed from a material having ahigher coefficient of thermal conductivity than the material from whichsaid cover member is made extending from said outer surface through saidcover member and said layer of insulation into said main body,substantially all of said insert members having an outer end lying in acommon plane with said outer surface; wear pad members mounted on saidouter surface of said cover member in said plurality of recesses atlocations spaced from said insert members, said wear pad members havingan outer surface lying in the plane of said outer ends of said insertmembers; and, extrusion orifices extending through said insert membersand forming continuations of said extrusion passageways of said mainbody.
 2. The extrusion die assembly of claim 1 wherein said insertmembers are positively joined to said cover member and said main body.3. The extrusion die assembly of claim 1 wherein a separate one of saidinsert members is associated with each of said passageways.
 4. Theextrusion die assembly of claim 1 wherein each said insert member isformed from sintered carbide.
 5. The extrusion die assembly of claim 1wherein said wear pad members are bonded to said cover member.
 6. Theextrusion die assembly of claim 1 wherein said insert members are bondedto said main body.
 7. The extrusion die assembly of claim 9 wherein saidinsert members have a generally circular configuration and saidextrusion orifices extend axially therethrough.
 8. A die assembly foruse in a plastic pelletizing apparatus of the type including an extruderfor extruding molten plastic and an associated cooling bath into whichextruded plastic pellets are discharged for cooling comprising:a rigiddie body having first and second spaced and oppositely disposed facesconnected by a plurality of extrusion passages; means for connectingsaid die body member to said extruder with said first face positioned toreceive molten plastic from said extruder; heating means for supplyingheat to said die body member; a layer of thermal insulation materialoverlying said second face; a cover member connected to said die bodymember to overlie said insulation and said second face, said covermember having an outer face directly exposed to the cooling bath and aplurality of recesses therein; a plurality of wear resistant insertmembers extending inwardly from the outer face of said cover memberthrough said cover member and said insulation into thermal conductivityengagement with said die body, discharge orifices formed through saidinsert members in communication with said extrusion passages forconveying molten plastic from the passages to the cooling bath, saidinsert members all having an outer end lying in a common plane; wear padmembers mounted on said outer face of said cover member in saidplurality of recesses at locations spaced from said insert members, saidwear pad members having an outer surface lying in said common plane;and, said insert members being formed from a material having acoefficient of thermal conductivity greater than the coefficient ofthermal conductivity of the material forming said cover member toconcentrate the flow of heat from said main body to said cooling baththrough the area surrounding said discharge orifices whereby thedischarge orifices are maintained at a higher temperature to reduce thepossibility of solidification and freezing-off of molten plastic as itpasses through said discharge orifices.
 9. The die assembly as definedin claim 8 wherein said heating means include passageway means formedthrough said body member and adapted to convey a heating fluid throughsaid die body.
 10. The die assembly as defined in claim 8 wherein aseparate one of said insert members is associated with each of saidpassages.
 11. The die assembly as defined in claim 8 wherein said outerend lies outwardly of the outer face of said cover member.
 12. The dieassembly as defined in claim 11 wherein said insert members each have aninner end that extends into said die body.
 13. The die assembly asdefined in claim 11 wherein said insert members are bonded to said diebody and said cover member.
 14. The die member assembly as defined inclaim 11 wherein said insert members each have a general cylindricalconfiguration with said discharge orifices extending axiallytherethrough.
 15. The die assembly as defined in claim 14 wherein saidinsert members have an inner end portion extending into said die bodyand are bonded therein.